I wouldn't limit that to developing nations, pretty much everyone I know uses WhatsApp in the UK, simply because mobile coverage is so poor that it's easier to

I wouldn't limit that to developing nations, pretty much everyone I know uses WhatsApp in the UK, simply because mobile coverage is so poor that it's easier to send a Whatsapp than it is to send an SMS. I seriously hope it doesn't cause any issue for these companies, I've just got back from a few months travelling and Whatsapp was invaluable.